1. Field of Invention
This invention generally relates to spectroscopy, particularly to monochromator systems for spectroscopy (e.g., for Raman spectroscopy).
2. Related Art
A number of techniques may be used to obtain information about materials. One technique that may be used is Raman spectroscopy. In Raman spectroscopy, laser light is incident on a surface of a material to be analyzed. Most of the light scatters elastically from the surface (which is referred to as Rayleigh scattering). However, some of the light interacts with the material at and near the surface and is scattered inelastically due to excitation of vibrational, rotational, and/or other low-frequency modes of the material. The inelastically scattered light is shifted in wavelength with respect to the incident laser light, either down in frequency (corresponding to the excitation of a material mode by the incident photons, also referred to as Raman Stokes), or up in frequency (corresponding to the interaction of the incident photons with an already-excited material mode, also referred to as an anti-Stokes Raman). The amount of the shift is independent of the excitation wavelength, and the Stokes and anti-Stokes lines are displaced from the excitation signal by amounts of equal magnitude.
Raman spectroscopy is performed by detecting the wavelength-shifted light. In order to detect light of a particular wavelength of interest, such as Raman-shifted laser light, a spectroscopy system includes a monochromator.
FIG. 1 shows a simplified example of a Raman spectroscopy system 100, according to the prior art. A laser source 110 illuminates a sample 120 mounted on a stage 130. Light 115 reflected from sample 120 includes elastically scattered light (which may be referred to as Rayleigh scattered light), as well as inelastically (Raman) scattered light. In order to isolate the Raman scattered light, system 100 includes a monochromator 105, including a diffraction grating 140, a filtering mechanism such as a notch filter 155 and/or slit 107, and a fixed detector 150. In order to analyze different regions of sample 120, stage 130 may be used to provide relative movement of the sample with respect to the incoming light.
Light 115 is incident on a rotatable diffraction grating 140, which disperses the light according to its wavelength. In FIG. 1, the relative position of grating 140 and detector 150 is selected to detect a desired wavelength λd, but not to detect other wavelengths λ1, λ2, and λ3. Because the Raman shift is relatively small, system 100 also includes a notch filter 155 positioned before detector 150 and configured to filter the strong Rayleigh scattered component at the excitation wavelength.
Different detector types may be used. In older spectroscopy systems, photomultiplier tubes (PMTs) were common. However, PMTs integrate the optical signal received on the entire detector surface. By contrast, newer spectroscopy systems generally use array detectors such as charge coupled device (CCD) array detectors, complementary metal oxide semiconductor (CMOS) detectors, and photodiode array detectors.
In order to detect the desired wavelength (and/or to scan a number of wavelengths), some existing systems rotate diffraction grating 140, while detector 150 is fixed. For example, if wavelengths λ1, λ2, and λ3 are of interest, diffraction grating 140 may be rotated to scan the range of wavelengths shown in FIG. 1.
For existing spectroscopy systems, the wavelength resolution for a particular measurement (i.e., the data collected at a particular rotation angle of diffraction grating 140) is fixed.
One way in which prior systems could be used to obtain more data about particular wavelength ranges of interest was to scan the light across the detector by rotating diffraction grating 140. For a low resolution system, a user could first scan the wavelengths rapidly, by rotating diffraction grating 140 through a first angular range at a first speed. After identifying the wavelength ranges of interest, the user could perform one or more additional scans. By performing the scans at a slower speed (and usually for a smaller angular range), the resolution of the spectroscopy can be increased.